In recent years, research and development on optical communication technologies that permit a realization of a higher transmission speed and a higher transmission capacity has been being carried out. One of these technologies is a WDM-PON technology that is a combination of a wavelength-division multiplexing (WDM) technology and a passive optical network (PON).
The WDM technology is an optical communication technology that multiplex transmits a plurality of optical signals of different wavelengths simultaneously via one (single-core) optical fiber. Transmission efficiency can be improved by use of the WDM technology because the plurality of optical signals are multiplex transmitted simultaneously via one optical fiber. Further, using the WDM technology permits establishing of an optical network with a high speed and high capacity at a low cost without increasing the number of optical fibers.
The PON is a form of an optical network in which a passive optical splitter is placed between an optical line terminal placed on a carrier side and a plurality of optical line terminals placed on a subscriber side. In the PON, the optical line terminal placed on a carrier side is connected to the optical splitter via one (single-core) optical fiber, and a plurality of optical fibers obtained by diverging by the optical splitter are connected to the plurality of optical line terminals placed on a subscriber side, respectively. The PON permits establishing of an optical network at a low cost because one optical fiber that is placed between an optical subscriber-terminal on a carrier side and the optical splitter is shared for a communication between the optical subscriber-terminal on a carrier side and the plurality of optical line terminals on a subscriber side.
In the WDM-PON that is a combination of the two technologies described above, different wavelengths are assigned to an optical signal transmitted by the optical line terminal on a subscriber side to the optical line terminal on a carrier side and an optical signal received by the optical line terminal on a subscriber side from the optical line terminal on a carrier side. Further, different wavelengths are assigned to those transmitted and received optical signals for each of the optical line terminals on a subscriber side. Thus, using the WDM-PON permits transmitting of an optical signal without interfering with other optical signals when one optical fiber is shared for transmitting a plurality of optical signals between each of the optical line terminals on a subscriber side and an optical line terminal on a carrier side. Further, the shared one optical fiber permits realizing of an optical communication with high speed and high capacity.
However, in the WDM-PON, when a failure has occurred in a transmission of an optical signal between an optical line terminal on a subscriber side and an optical line terminal on a carrier side, a replacement with a spare unit that covers the same wavelength as the wavelength in which the failure occurred is needed so as to recover from the failure.
The number of combinations of wavelengths of optical signals transmitted and received between an optical line terminal on a subscriber side and an optical line terminal on a carrier side increases as the number of optical line terminals that are placed on a subscriber side increases. Thus, preparing in advance all spare units for all the optical line terminals on a subscriber side and on a carrier side leads to an increase in costs for maintenance and operation for an optical communication system.
Further, a mistaken replacement of an optical line terminal on a subscriber side or on a carrier side with a spare unit that covers a different wavelength will induce a failure in other optical lines that have no reason to fail.
The following is known as a technology relating to the WDM-PON technology. Namely, an optical transmission terminal modulates a signal in the upstream direction that deals with the information from the optical transmission terminal side, and converts the upstream signal obtained by the modulation into an upstream optical signal of a designated wavelength. The optical transmission terminal supplies the upstream optical signal obtained by the conversion to a first optical star coupler via a second optical star coupler. The first optical star coupler multiplexes the upstream optical signals supplied from a plurality of optical transmission terminals for wavelength multiplexing, and supplies the wavelength multiplexed upstream optical signal to an optical subscriber-line terminal. Further, the optical subscriber-line terminal modulates a downstream signal that deals with the information of the optical subscriber-line terminal side, and photoconverts the downstream optical signal obtained by the modulation with a wavelength different from those of the multiplexed upstream optical signal and the other downstream optical signals. The optical subscriber-line terminal supplies the photoconverted downstream signal to the first star coupler via a third optical star coupler. The first star coupler diverges the multiplexed downstream optical signal for each path and supplies the downstream signals obtained by the diverging to the respective optical transmission terminals.
The following technology is also known. Namely, sets of wavelengths (λu1, λd1) and (λu2, λd2) that are a pair of a wavelength of an upstream signal and a wavelength of a downstream signal are assigned to each PON system that transmits a signal between a center side unit (OLT) and a subscriber side unit (ONU) via a plurality of optical transmission paths. In a PON system, a working path and a spare path are set via two sets of wavelengths (λu1, λd1) and (λu2, λd2), and communications through a working path and a spare path are always established. Dynamic SWs placed opposite to the OLT and the ONU always confirm connectivity by a continuous check (CC) on working and spare connections. When a failure has occurred on the working connection, a switching from the working connection to the spare connection is performed by the opposite dynamic SW.